There is at present a wide selection of technology that can be used for desorbing materials from resins. Some technologies are better suited than others for particular applications and, therefore, selecting the most appropriate technology is an important factor in achieving a high desorption rate and cost effectiveness.
Generally speaking desorption processes for desorbing material from a resin may be carried out as either batch or continuous operations which usually corresponds to the apparatuses for carrying out processes having either so-called fixed-beds or moving-beds.
Apparatuses with fixed-beds are at present the most widely used in industry. For example, a text by Abrams I. M. entitled “Type of ion-exchange systems” (Ion Exchange for Pollution Control, eds. C. Calmon and H. Gold, CRC Press, Boca Raton, vol. 1, pp. 71-850, 1979) describes that fixed-bed equipment items have been operated for more than. 25 years and are still presently in use for softening 1500 mega-liters/day of water at the Metropolitan Water District of Southern California.
A text by Salem E. entitled “Equipment operation and design” (Ion Exchange for Pollution Control, eds. C. Calmon and H. Gold, CRC Press, Boca Raton, vol. 1, pp. 87-100, 1979) describes that the desorption cycle of most fixed-bed apparatuses involves: firstly backwashing a bed of full loaded or saturated resin; settling the bed; feeding desorption solution through the bed; displacement of desorption solution (or slow rinse); and finally rinsing the resin before supplying a pregnant solution to the bed again.
The backwashing stage removes suspended particles, which have accumulated within the resin bed and eliminates channels that, may have formed during the sorption stage. Backwashing also helps to break up agglomerates formed between suspended particles and the ion-exchange resin.
The settling stage follows the backwashing stage and is important to avoid channeling of fluid through the bed.
Desorption is accomplished by passing desorption solution through the bed to convert the resin to the desired form. After an adequate volume of desorption solution has made contact with the resin, displacement of desorption solution from the bed takes place.
Rinsing of the resin with demineralised water is normally used to remove the last residues of desorption solution from the bed.
Upon completion of the rinsing stage, the liquid phase containing targeted material to be sorbed into the resin during a sorption stage enters at the top of the column when the column is operated in co-current or at the bottom of the column when the column is operated in countercurrent.
U.S. Pat. No. 4,412,866 describes a modification of a batch-fixed bed process and in particular relates to a simulated moving-bed in which separate zones are defined, each of which include one or more discrete vessels. The zones correspond to the functions of the process; typically sorption, displacement, desorption and rinsing. Booster pumps connected in series with the vessels maintain a desired pressure head for each zone. The functions of each zone are rotated in sequence, the sequence being timed in relation of the migration of the front between adjacent phases in the fluid loop circulating through the zones.
Another type of absorption/desorption processes is a continuous process. Generally speaking an absorption/desorption process is classified as a continuous process when sorption, rinsing and desorption are conducted simultaneously and the product flow is uninterrupted. The use of a moving bed of resin allows one to obtain continuous operation and the main advantage is the high processing efficiency.
As with batch processes, continuous processes can be operated as either co-current or countercurrent.
Not all processes described as continuous are truly continuous processes. Truly continuous processes operate without interruption of either resin or liquid flows. Semicontinuous processes are often characterised by a short residence period in which ion-exchange absorption occurs (i.e. the service mode) followed by a period when the resin bed is moved (the moving mode). However, because the periods for both modes are very short, the processes virtually behave as a continuous one. More than a hundred semicontinuous processes are known, but only about six have any real industrial significance.
To our understanding the widest known process of this type is the so-called Higgins Loop (and is described in the text by Higgins, I. R. and Roberts, I. T. “A countercurrent solid-liquid contactor for continuous ion-exchange”. Eng Prog. Symp. Ser., 50, 87-94, 1950). The Higgins Loop is a continuous countercurrent ion-exchange process for liquid phase separations of ionic components using solid ion-exchange resin.
The Higgins Loop comprises a vertical cylindrical vessel containing a packed-bed of ion-exchange resin that is separated into four operating zones by butterfly or loop valves. These operating zones—adsorption, desorption, backwashing and pulsing—function like four separate vessels.
The Higgins Loop treats liquids in the sorption zone with resin while the ions are removed from loaded resin in the desorption zone simultaneously. Intermittently, a small portion of resin is removed from the respective zone and replaced with stripped or loaded resin at the opposite end of that zone. This is accomplished hydraulically by pulsation of the resin through the loop. The result is a continuous process that contacts liquid and resin in countercurrent flow.
It is an object of the present invention to provide an alternative method and apparatus for desorbing materials sorbed on a resin that is capable of producing a concentrated eluate stream.